In the spray drying of products which at certain combinations of temperatures and moisture contents are sticky, various processes and apparatuses are used to obtain the product as an agglomerated powder.
In certain of the spray drying systems used for this purpose the atomization of the liquid to be dried and at least a substantial part of the drying of the atomized liquid take place in a chamber of which the lower part is formed by a conical downward tapering section, and the liquid is atomized into a central downward stream of drying gas, whereas the spent drying gas is recovered together with entrained fine powder particles from non-central locations in the uppermost part of the chamber.
In the present specification the term "central" indicates locations on or near the vertical centeraxis of the chamber, whereas the term "non-central location" means any place between the central one and the periphery of the chamber. The above mentioned recovering of spent drying gas may thus for instance take place through an annular orifice surrounding the central downward stream of drying gas.
The product comprising agglomerated powder particles of suitable size is recovered from the bottom part of the conical section of the drying chamber.
The fine particles entrained with the spent drying gas and often forming a substantial part of the particles produced in the spray drying process are recovered by means of cyclones, bag filters or other conventional dust-separating equipment.
Since said fine particles will often be too small to fulfil the specifications set for the final product they are recycled to the chamber to contact wet droplets or sticky moist particles to form agglomerates of suitable size.
Said recycling are conventionally made to locations in the upper part of the spraying chamber, for instance in adjustable distance from the atomizing device, as disclosed in European patent application No. 378 498, alternatively the fine particles are injected tangentially along the chamber walls in the lower part of the chamber. It has also been suggested to inject the fine particles in an upward direction just below the atomizer device to have the fine particles intersect the part of the droplets ejected from the atomizer device, confer U.S. Pat. Nos. 3,324,567, 3,460,600 and 4,279,873. However, this last mentioned system for recycling of the fine particles has been suggested only in connection with spray driers having a flow pattern substantially different from the one existing in the above described drying chambers in which the drying air is introduced in a central downward stream and leaves the chamber from non-central exits in the top of the chamber.
Among the spray drying processes of special interest in connection with the present invention are those, in which a layer of fluidized particles is maintained in the bottom of the spray drying chamber. Said fluidized layer receives partly dried particles formed by partial drying of droplets atomized in the upper part of the chamber. In the fluidized layer these spray dried particles are agglomerated and further dried either to the final moisture content or to a moisture content above the desired moisture content in the final product, but sufficiently low to enable recovering of the agglomerated powder for subsequent drying, for instance in a vibrated fluidized bed apparatus outside the drying chamber. A process of this type is described for example in European patent specification No. 97 484 and the corresponding U.S. Pat. No. 4,490,403, incorporated herein by reference.
In the prior art systems having a fluidized layer in the bottom of the spray drying chamber, the fine particles separated from the spent drying gas are usually recycled directly to the fluidized layer for further agglomeration therein.
The present invention provides improvements in spray drying and agglomeration processes utilizing the typical flow pattern formed in a spray drying chamber, in which the centrally downward introduced drying air is withdrawn from non-central locations in the top of the spray drying chamber.
Such recovering of the spent drying gas from non-central locations in the top of the drying chamber is mainly, but not solely utilized in connection with systems having a fluidized layer in the bottom of the chamber, for which reason an essential feature of the invention is improvement of such processes and apparatuses utilizing an integrated fluid bed.
It is an object of the invention to improve the product quality of processes of the above described type to obtain desired large agglomerated particles having a low content of dustforming small particles and having a very low degree of heat damage.
An other object is to increase the production capacity and the heat economy of spray drying apparatuses and processes of the above defined type and to facilitate operation thereof.
Further objects and advantages will appear form the below description.